Method for improving wear characteristics of bronze

ABSTRACT

THE METHOD OF IMPROVING THE WEAR OF DURABLILITY CHARACTERISTICS OF BRONZE BY HEATING THE BRONZE TO A TEMPERATURE BETWEEN APPROXIMATELY 968*F. AND 1470*F., HOLDING THE ATTAINED ELEVATED TEMPERATURE FOR A PERIOD OF TIME, AND THEN SUBSEQUENTLY COOLING THE BRONZE. THE BRONZE MAY BE HELD AT THE ELEVATED TEPERATURE FOR A RELATIVELY SHORT PERIOD OF TIME SUCH AS, FOR INSTANCE, 15 SECONDS. THE COOLING STEP MAY INCLUDE AIR COOLING THE BRONZE DOWN TO APPROXIMATELY 700*F. TO 800*F. AND THEN LIQUID QUENCHING THE BRONZE DOWN TO ROOM TEMPERATURE.

July 16; 1914 R. L. WOLFE ETAL 3,824,132

III-ITHOD FOR IMPROVING CHARACTERISTICS-OF BRONZE Filed July 21, 1972 INVENTORS ROBERT L. WOLFE AUGUSTIN E TREBNIK ATTORNEYS United States Patent Office 3,824,132 Patented July 16, 1974 3,824,132 METHOD FOR IMPROVING WEAR CHARACTERISTICS OF BRONZE Robert L. Wolfe and Augustine Trebnik, Columbus, Ind., gslslignors to Reliance Electric Company, Cleveland,

Continuation-impart of abandoned application Ser. No. 66,391, Aug. 24, 1970. This application July 21, 1972, Ser. No. 274,114

Int. Cl. C221? 1/08 U.S. Cl. 14813.2 13 Claims ABSTRACT OF THE DISCLOSURE The method of improving the wear or durability characteristics of bronze by heating the bronze to a temperature between approximately 968 F. and 1470 F., holding the attained elevated temperature for a period of time, and then subsequently cooling the bronze. The bronze may be held at the elevated temperature for a relatively short period of time such as, for instance, 15 seconds. The cooling step may include air cooling the bronze down to approximately 700 F. to 800 F. and then liquid quenching the bronze down to room temperature.

This is a continuation-in-part application based on our prior co-pending application Ser. No. 66,391, filed Aug. 24, 1970, now abandoned based on parent application Ser. No. 755,902, filed Aug. 28, 1968, now U.S. Pat. No. 3,557,423.

It is a primary object of our invention to provide a method for improving the wear or durability characteristics of bronze. A secondary object of our invention is to combine such a method with a method for joining a bronze gear ring to a steel hub. Particularly, we have provided a method for joining the bronze portion of the gear to the steel portion of the gear while, at the same time, heat treating the bronze to improve the wear or durability characteristics. Of course, as is conventional, the gear teeth are formed in the bronze portion of the gear, thereby to take advantage of the characteristic of bronze which permits it to mate with a ferrous metal.

Ordinarily, bronze is not heat treated and is merely used as cast, though certain aluminum bronzes are heat treated to increase their tensile strength. We have discovered that the heating and subsequent cooling involved in our method for joining the bronze portion of a gear to the steel portion of a gear does significantly improve the pitting wear characteristics of the bronze. Of course, by significantly improving the pitting wear characteristics of the bronze portion, i.e., the portion in which the gear teeth are formed, we have provided a significantly improved gear.

To our knowledge, no one has used our method for fabricating a steel-bronze composite gear and, further, no one has used or preferred heating and cooling steps to heat treat bronze to improve its pitting wear characteristics.

In the past, steel-bronze composite gears have been fabricated by casting a bronze annulus about the outer periphery of a steel annulus. This method is commonly referred to as chill casting. The chill cast bronze annulus is not fused to the outer periphery of the steel annulus and, therefore in some cases, it has been necessary to serrate or groove the outer periphery of the steel annulus so that the chill cast bronze annulus will remain in position on the steel annulus.

Steel-bronze composite gears have also been fabricated by welding the parts or by using screws or other fastening devices to hold the outer bronze annulus in position on the inner steel annulus.

Our method involves the use of an intermediate material to join the outer bronze annulus to the inner steel annulus. Such an intermediate material is commonly referred to as solder. We prefer to use a solder which, when heated to a point above its melting temperature and then cooled below such a point, will fuse to the bronze and steel annuli.

Our method may be used to join a bronze annulus to a cast iron hub to produce a composite iron-bronze gear.

It is an object of our invention, therefore, to provide a method of fabricating an improved composite gear which comprises the steps of arranging an inner ferrous metal and an outer bronze annulus to define a concentric annular space therebetween, placing a solder member adjacent the annular space, heating the annuli and the solder member concurrently, while maintaining the positional relationship of the annuli, to cause molten metal from the solder member to flow into and fill the annular space, and then, while still maintaining the positional relationship of the annuli, cooling the annuli to a temperature below the melting point of the solder member, thereby to join the annuli and to improve the pitting wear characteristics of the bronze annulus. Of course, the solder member must have a composition whose melting point is significantly lower than that of either annulus, i.e., lower than the melting point of the ferrous metal or of the bronze.

Another object of our invention is to provide such a method in which the annuli are heated to a temperature between approximately 968 F. and 1470 F. and then cooled in such a manner as to improve the pitting wear characteristics of the bronze annulus. The preferred solder will melt in the temperature range of 1100" F. to 1175 F., although there are some solders which will melt at temperatures as low as 600 F. and still provide a strong bond.

Another object of our invention is to provide such a method in which the annuli are heated during the heating step to a temperature ranging from 1100 F. to 1260" F.

Another object of our invention is to provide such a method in which, after the annuli. are heated to a temperature ranging from 1100 F. to 1260 F., the cooling step includes air cooling the annuli down to approxi mately 700-800 F. and then water quenching rapidly down to approximately room temperature.

Another object of our invention is to provide such a method in which the heating step includes placing the annuli and the solder member in a furnace held at approximately 1400 F. until the solder member melts and for a predetermined time thereafter, then removing the annuli from the furnace. We presently believe that such a predetermined time should be a relatively short period such as, for instance, 15 seconds.

Still another object of our invention is to provide such a method in which, after the annuli are removed from the furnace, the cooling step includes air cooling the annuli down to approximately 700-800 F. and then water quenching the annuli down to approximately room temperature.

It will be appreciated by those familiar with metallurgical arts, and particularly bronze heat treating arts, that a relatively short heat treating period such as 15 seconds at the temperature range (1100 F. to 1200 F.) at which solder melts is quite novel. It is to be compared, for instance, with a report in Chill-Cast; Tin Bronzes, Arnold & Co., London, 1951, Heat Treatment of Cast Bronze on page 305 where the authors reported investigating theetfect of annealing for periods of ,41, to 24 hours at 600 C. (1112 F.). The time of 15 seconds is, of course, onesixtieth of the time of 15 minutes.

It has been discovered that applicants relatively short heat treating process will improve copper-tin, bronzes cast to have a substantial delta phase constituent of, for instance, between approximately 2% and 7% in an alpha phase matrix. Such copper-tin bronze, as cast, contains such a delta phase constituent in the form of delta phase crystals supported in the alpha phase martix. One such bronze is S.A.E. 640 bronze which conventionally includes 10 to 12 percent tin and which conventionally is cast in such a manner as to provide a substantial delta phase constitutent. For instance, conventional chill casting processes and continuous casting processes which involve chill casting will produce a substantial delta phase constituent if the bronze melt contains sufficient tin. It is the tin content and the method of casting, and particularly the cooling time involved in the casting process, which produces the substantial delta phase constituent.

Each delta phase crystal, of course, is formed with sharp edges and sharp corners which produce stress concentration and stress risers in the alpha phase matrix. The present invention involves heat treating such bronze for relatively short periods of time at temperatures which will be sufficient to diffuse or start diffusing the delta phase crystals. Importantly, the time is kept short so that the heat treating, in accordance with the present invention, merely diffuses off or rounds off the sharp corners and sharp edges of the delta phase crystals to reduce or to eliminate the stress risers in the alpha phase matrix produced by the sharp edges and sharp corners.

As will be more fully discussed hereinafter, bronze wear elements, such as bronze gears and bronze bushings, often fail because of pitting Wear and not because of the more commonly known abrasive wear. Pitting wear occurs when there is surface fatigue. Before the process of the present invention, the as cast copper-tin bronze included delta phase crystals having sharp edges and sharp corners producing stress risers and stress concen trations in the alpha phase matrix. Under load and fatigue the risers and concentrations caused fissures and cracks which led to surface fatigue or pitting wear failure.

Therefore, it is an object of the present invention to heat treat the as cast copper-tin bronze only for the purpose of diffusing off or rounding off the sharp corners and sharp edges of the delta phase crystals. The presence of the delta phase crystals is well known to be essential to the bronze and they should not be eliminated during the heat treating process because it is the delta phase crystals which give the bronze good abrasive wear resistance. If the crystals are eliminated or substantially eliminated, the abrasive wear resistance will be greatly deteriorated. Thus, it is an object of the present invention to provide a heat treating cycle which will increase the resistance to surface fatigue or pitting wear without substantially deteriorating the abrasive Wear resistance of copper-tin bronze.

Continuously cast copper-tin bronze ordinarily will contain between 2% and 7% delta phase constituent. The preferred range is above 4% delta phase constituent. It is believed that, preferably, the heat treating process should not reduce the total delta phase constituent by more than approximately to 50% of that constituent existing in the as cast condition. For example, a cast copper-tin bronze which contains a 4% delta phase constituent may be successfully heat treated in accordance with the present invention to diffuse off the sharp corners and sharp edges of the delta phase crystals without reducing the delta phase constituent to a value less than 3%. A further reduction might well improve the pitting wear resistance, but it could very well deteriorate the abrasive wear resistance of the bronze.

Now, the objective of merely diffusing off the sharp corners and sharp edges of the delta phase crystals without eliminating or substantially eliminating the crystals is accomplished by controlling the time at which the as cast copper-tin bronze is held in the range of temperature at which diffusion will occur. It is believed that this range runs from approximately 968 F. to approximately 1470 F. Below 968 F., it is believed that diffusion will not occur and above 1470 F. it is believed that partial melting will occur. Further, the diffusion rate at the temperatures approaching 1470" F. is so great that processes involving such temperatures are difficult to control, i.e., the time factor is extremely critical. When lower temperatures, for instance, in the range of 1100 F. to 1260 F., are used, the diffusion rate is slow enough to permit reasonable tolerances in the time factor and yet produce satisfactory results. Since, as will be pointed out hereinafter, the diffusion rate is believed to double for each approximately 33 increase in temperature, a suitable attained elevated temperature range may be 1100 F. to 1200 F. It will be appreciated that this temperature range correlates with the aforesaid temperature range at which the preferred silver solder melts, i.e., about 1100 F. to 1175 F. Further, if the heat treatment process does not involve solder or the use of solder which melts at temperatures above 1100 F., then a suitable temperature range may have a lower limit of approximately 1000 F. which is comfortably above the 968 F. temperature at which diffusion starts. Thus, depending upon available heating equipment and control equipment, in accordance with the present invention, the attained elevated temperature may suitably range from approximately 1000" F. to approximately 1260 F.

It will be appreciated that, when the attained elevated temperature is significantly above 968 F., the time involved in heating the bronze up to the attained elevated temperature from 968 F. and cooling it down to 968 F. from the elevated temperature must be considered because difiusion will be occurring at all times above the 968 F. basic temperature. Therefore, the process of the present invention, from a practicability standpoint, involves three time periods including T which is the time required to heat the bronze from 968 F. to the elevated temperature, T which is the dwell time at the attained elevated temperature, and T which is the time required to cool the bronze from the elevated temperature to below 968 F. For process control purposes, the time T at the attained elevated temperature usually will be the most critical to control since the diffusion is fastest at that temperature. Further, if the mass and shape of the bronze part and the characteristics of the heating equipment are already established, the time T may be the only time which can be controlled, the time T being determined by the heat absorbing characteristics of the bronze part and the capacity of the heating equipment and the time T being determined by the heat loss characteristics of the part.

As will be more fully explained hereinafter, a formula may be used to determine the most appropriate elevated temperature and the times, T T and T The method of the present invention, therefore, involves a short time excursion into the diffusion range to achieve a smoothing of the external surfaces of the delta phase crystals to improve the pitting wear characteristics of the bronze without significantly deteriorating the abrasive wear characteristics of the bronze provided by the presence of such crystals.

Other objects and features of the present invention will become apparent as this description progresses.

To the accomplishment of the above and related objects, the method of the present invention may be practiced as described herein and as illustrated in the accompanying drawings, attention being called to the fact, however, that the description and drawings are illustrative only, and that change may be made in the specific methods illustrated and described, so long as the scope of the appended claims is not violated.

In the drawings:

FIG. 1 is an elevational View of a steel-bronze composite gear blank, i.e., a blank in which the gear teeth are subsequently formed, and showing the manner in which we place a solder ring relative to the bronze and steel annuli before they are heated; and

FIG. 2 is a sectional view taken from FIG. 1 generally along the line 2-2.

Referring now to the drawings, it will be seen that we have illustrated a composite gear blank comprising a steel annulus 12, hereinafter referred to as the hub 12, and a bronze annulus 14, hereinafter referred to as the ring 14. Gear teeth (not shown) may be formed in the ring 14 after it is joined to the hub 12 by our method which will be described hereinafter.

It will be appreciated that our method is particularly suited for fabrication of worm gears because the teeth of a worm gear are engaged frictionally by the thread of its associated worm. Thus, it is desirable to form the worm gear teeth in a bronze metal. We prefer to form the ring 14 from S.A.E. 640 bronze, the composition of which is copper 85-88%, lead l1.5%, phosphorus 0.2- 0.3%, zinc 0.5%, silicon 0.005%, nickel 0.751.5%, iron 0.3%, aluminum 0.005% and tin 10-12%. This bronze is hard, strong, tough, resistant to Wear and easy to machine. We have found that it is convenient to obtain a continuously cast S.A.E. 640 bronze sleeve and to slice the sleeve into sections to provide the rings 14. The continuously cast S.A.E. 640 bronze sleeve has a substantial delta phase constituent because the casting process involves casting the molten bronze through a chilled die.

Further, S.A.E. 640 bronze includes five basic elements or constituents of copper, tin, phosphorus, lead and nickel. The other elements of aluminum, zinc and iron are said to be tramp elements i.e., elements which are not required, but which are permissible, perhaps as contamination elements. For example, aluminum is permissible up to 0.005%, zinc is permissible up to 0.5% and iron is permissible up to 0.3%.

It will be appreciated that other types of copper-tin bronze may be suitable for use as a gear material and for treatment by the method of the present invention as long as the bronze includes suflicient tin and is cast in such a manner as to form a substantial delta phase constituent. A bronze including 10 to 12% tin and chill cast or continuously cast will include a substantial delta phase constituent of, for instance, 2% to 7% in the alpha phase matrix.

In order to provide a bronze having suitable gear characteristics, a bronze including 85 to 88% copper, 10- 12% tin, and up to .3% phosphorus with a substantial delta phase constituent as cast may be used. Such a bronze may also include, as a desirable constituent up to 1.5% lead to enhance its machining properties.

The hub 12 is provided with a conventional center opening 16 and an axially extending keyway 18, the opening 16 being bevelled as indicated by the reference numerals 20 in FIG. 2. After the ring 14 is joined to the hub 12, the gear teeth may be formed in the ring in a conventional manner. We prefer to form the teeth after the joining operation in order to assure that the teeth will be formed concentrically with the opening 16. Methods of forming gear teeth are well known and need not be discussed, in detail, in this description.

We prefer to form the hub 12 with a radially outwardly and peripherally extending flange portion 22 against which the ring 14 abuts as shown in FIG. 2. This flange portion 22 positions the ring 14 axially relative to the hub 12. Further, we prefer to form the hub 12 and ring 14 so that the diameter of the outer periphery 24 of the hub is from approximately .001 to .005 inch less than the diameter of the inner periphery 26 of the ring. Thus, when the ring 14 and hub 12 are concentrically arranged as illustrated in FIGS. 1 and 2, there is a concentric annular space defined by the outer periphery 24 and the inner periphery 26 between the hub 12 and ring 14. Of course, this space between the outer periphery 24 and inner periphery 26 is very slight and, by a radial dimension, may be as small as .0005 inch. Nevertheless, it is into this space that we cause molten solder to flow, which solder, when it hardens, joints the bronze ring 14 to the hub 12. This space must be sutficient to permit the two parts to be assembled at room temperature, but should be as small as feasible, since a heavy layer of solder in the finished product would tend to weaken the composite gear.

It is necessary, of course, to use a fluxing agent on the outer periphery 24 and inner periphery 26 in order to get the solder to flow into and completely to fill the space defined by the said outer and inner peripheries. It is conventional to use flux in soldering operations and we use a commercially available flux and apply it in a conventional manner. For instance, we have used a hightemperature silver solder, which is sold commercially under the name Handy Harmon Easy Flow 45 and which melts at about 1100 F. to 1175 F. With this silver solder, we use Handy Harmon Flux. When we use this solder and properly flux the outer and inner peripheries 24, 26, the solder is drawn into the space between the peripheries when the hub 12 and ring 14 are heated to about 1100" F. to 1175 F.

The fit between the hub 12 and the ring 14, i.e., the .001 to .005 inch tolerance described above, is designed to minimize the amount of solder required to join the ring 14 to the hub 12, the amount of solder being minimized because of strength considerations as well as cost considerations. Further, the small space between the to a capillary action, which draws the solder generally peripheries 24, 26 provides an action, which is analogous to a capillary action, which draws the solder generally uniformly to fill the space.

In order to position the solder in such a manner that it will be able to flow into and to fill uniformly the space between the peripheries 24, 26, we prefer to provide a solder ring which has a mean diameter approximately equal to the diameter of the outer periphery 24, the upper end of the hub 12 being bevelled as shown and the thickness of the ring being, for instance, of an inch. We have shown such a ring 30 in FIG. 2. Referring to FIG. 2, it will be seen that the ring 30, the axis of which coincides with the axis of the hub 12, spans the upper adjacent edges of the outer and inner peripheries 24, 26. We place the hub 12, ring 14 and ring 30 in a heating means with the coinciding axes of the hub and ring generally vertical. In such a case, the outer peripheral surface 24 and the inner peripheral surface 26 extend generally vertically.

It will be appreciated that we can lay the hub 12 on a support means with the flange portion 22 down and then slip the ring 14 downwardly over the bevel of the hub to engage and be supported. by the flange portion 22. Since, as illustrated in FIG. 2, the hub 12 is slightly thicker than the ring 14, the solder ring 30 can be placed about the upper peripheral edge of the hub to be held in registry with the space between the peripheries 24, 26. Thus, by using the structure illustrated in FIG. 2, We are not required to use any special jigs or fixtures to hold the hub 12, ring 14 and solder ring 30 in the proper positional relationship.

We have used successfully a low frequency induction heating unit to heat the hub 12, ring 14 and solder ring 30. The unit we have used is sold commercially under the trademark Tocco. We simply place the hub 12, ring 14 and solder ring 30 in the induction heating unit as discussed in conjunction with FIG. 2 and then heat the hub and rings until the solder melts and flows downward- 1y into the space between the peripheries 24, 26, and continue heating for a predetermined period, such as fifteen seconds, thereatfer. j

. We have performed the heating operation in a muffle furnace which is held at a temperature of approximately 1400 F. When we use the muffle furnace at this temperature, the hub 12 and ring 14 are withdrawn from the furnace approximately fifteen seconds after the solder melts. Since the solder We prefer to use will begin to melt at approximately 1100 F. to 1175 F., it will be appreciated that there is more than sufficient heat required to melts. Since the solder we prefer to use will begin to melt into the space between the peripheries 24, 26. From this work with the muffle furnace, it appears to be practical to heat the hub 12, ring 14 and solder ring 30 in a highvolume, automatic gas furnace.

Because of the extremely effective and uninterrupted bond between the steel core and the bronze annulus which results from our process, we are able to minimize the radial thickness of the annulus of expensive bronze, thereby further reducing the cost of the finished composite gear. Whereas, in previous composite worm gear constructions known to us, the need for serrations at the interface, or for solid material to accommodate threading for set screws or the like, has required a very substantial thickness of bronze between the roots of the gear teeth and the inner surface of the annulus, we have found that we can achieve wholly satisfactory results when the radial thickness of the annulus exceeds the depth of the tooth by only a few thousandths of an inch, so that only a thin web of bronze spans the tooth roots.

Of course, the heating step of our method must involve temperatures which are less than the melting point of bronze which is approximately 1922 F. We initially believed that this temperature of 1922 F. was our only limitation and that we might heat the bronze ring 14 up to, for instance, 1600 F. and perhaps even to 1700 F. or 18-00 F. We now understand that the upper limit is approximately 1470 F. Since our method involves one heating step to accomplish both a soldering function as well as a bronze heat treating function, it is necessary for us to optimize our controls to obtain satisfactory soldering as well as improved pitting wear characteristics of the bronze. Our initial work led us to believe that our preferred heating range will be between 1100 F. and 1500 F. Prior to filing this continuation-in-part application, it has been determined that the upper limit is approximately 1470 F., above which partial melting occurs. This range (1100 F. to 1470 F.) permits the use of high-temperature solder and, in addition, gives the desired improved wear characteristics.

We have found that it is desirable, after the abovedescribed heating steps, to air cool the hub 12 and ring 14 down to approximately 700-800 F. and then to water quench the hub and ring down to approximately room temperature. However, we can omit the quench step and air cool the hub 12 and ring 14 down to room temperature and obtain, at least, comparable results as to durability of the bronze. It will be appreciated that the quenching step does speed up the process.

In the aforesaid furnace process where the hub 12 and ring 14 are withdrawn approximately 15 seconds after the solder melts, the only time period controlled is the dwell time T Since the temperature of the furnace (1400 F.) is higher than the temperature at which the solder will melt (1lO0-1175 F.), the temperature of the ring will be increasing during the 15 second dwell time, probably within the llO-ll75 F. range. In such a process, the times T and T were established by, for instance, the mass and surface area of the hub 12 and ring 14, i.e., their ability to absorb heat during the T time period and to radiate heat during the T time period. It has been found that bronze rings placed in a gas fired furnace preheated to 1400 F. may be heated from room temperature to approximately 1100 F. in 7 to 9 minutes and that bronze rings may be heated to approximately 1100 F. in the Tocco induction heater in approximately 4 to 8 minutes.

It has been found that a thick bronze test ring having an internal diameter (I.D.) of 1% and an outer diameter (CD) of 3%" can be heated from 968 F. to 1100 F. in the Tocco induction heater during a T time period of approximately 1% minutes. That ring cooled in air from 1100 F. to 968 F. during a T time period of approximately /3 minute. It has been found that a ring of the same size can be heated from 986 F. to 1200 F. in the Tocco induction heater during a T time period of approximately 2 /2 minutes and then cooled by air from 1200 F. to 968 F. during a T time period of approximately /2 minute. It will be appreciated that the T time period will be independent of the heating method.

The dwell time T at the attained elevated temperature, or the attained elevated temperature range if a specific temperature cannot be held, therefore, may be varied to give the total desired diffusion. The appropriate dwell time T and the appropriate elevated temperature or elevated temperature range to be maintained during the dwell time may be arrived at by experimentation and/or calculation.

It is usually advantageous to determine a time-temperature profile for heating and cooling a given bronze specimen using a particular heating device. Thermocouples may be attached to the specimen and placed in the heating device. Then, the time-temperature profile for that particular specimen in that particular device may be established by monitoring and recording the temperature with respect to time. Particularly, it is desirable to record the time T that it takes for the specimen to heat up from 968 F. to the desired elevated temperature and then the time T required for the specimen to cool from the elevated temperature to 968 F. Preferably, for reasons which will become apparent as this description progresses, the time and temperature readings should be established for each incremental temperature increase of, for instance, 33 F.

Once the time-temperature profile is established, the diffusion effect profile (DEP) can be established by dividing the temperature into zones of, for instance, approximately 33 F. It has been reported and it is generally accepted that the theoretical diffusion rate doubles from zone to zone as the temperature rises if each zone is approximately 33F. Therefore, a relative diffusion factor (RDF) can be assigned to each zone. The diffusion effect profile (DEP) can then be equated to the product of the relative diffusion factor (RDF) and the time (T).

Table I below shows an arbitrary scale for the relative diffusion factor (RDF) for each approximately 33 zone from 968 F. to 1233 F.

A review of the Table I will show that arbitrarily an RDF of 0.7 has been assigned to the zone A between 968 F. and 1000 F. The RDF for each successive zone, therefore, is obtained by doubling or approximately doubling the RDF of the subjacent zone.

The DEP for each zone can then be calculated by multiplying the RDF for that zone by the time duration for that zone as determined by the temperature-time profile tests. Then the total DEP can be obtained by adding the DEP for each zone.

Table 11 below shows the DEP for each zone and the total DEP obtained by placing a bronze specimen in a 1200 F. lead pot and then air cooling the specimen to 968 F. The sum of the DEP in Table II is 1732. It will be appreciated by those familiar with heat treating arts that heat treating in a molten lead pot is extremely fast as compared to, for instance, a gas fired furnace. In the example of Table II, the specimen was left in the 1200" F. lead pot for 1 minute and then air cooled down to 968, F.

TABLE II RDF X T DEP Sum 1,732

Although, in the example of Table II, it is believed that too much delta phase is diffused, the example is suitable for the purpose of showing how the DEP may be calculated. Table III below shows the DEP calculations involved in heating a bronze specimen by placing it for 2 minutes in an 1100 F. lead pot and then letting it air cool to 986 F. while monitoring the temperature of the specimen. The total DEP in Table III is 678 or 1054 less than the above Table II figure of 1732. Thus, in the example of Table III, less delta phase constituent was diffused into the alpha phase matrix.

TABLE III RDF X T DEF Sum 678 Table IV below shows the results of placing a bronze specimen in a 1000 F. lead pot and leaving it there for 16 minutes and then air cooling it down to 968 F.

TABLE IV RDF X T DEP Sum 690 It will be appreciated that the T time and T time for the example of Table IV and the diffusion occurring during those periods, would be extremely small as compared to the diffusion occurring during the 16 minute period. Thus, the only significant figure in the example of Table IV is obtained by multiplying the RDF by the total time of 984 seconds (16 minutes) to obtain a total DEP of 690.

It has been found that the diffusion rate at the lower temperatures within the range of 968 F. to 1470 F. is quite slow. The variations in DEP obtained in the examples of Tables II, III and IV bear this out.

It will be appreciated that the calculation of DEF by zones is not an exact or precise technique and that the Y 1'0 zones is an approach to solving the following integral equation or formula:

elevated temperature range In the above formula RDF is an arbitrary scale based upon established expectable diffusion rates of delta phase constituent into the alpha phase matrix.

It will be appreciated that the RDF for various temperature zones can also be readily experimentally determined by subjecting bronze specimens having known delta phase constituents to heat at temperatures within the various zones and then determining how much delta phase has been diffused from the specimens. Such a determination may be made using conventional metallographic techniques. As discussed in conjunction with Table I, it is understood in the art that the diffusion rate doubles for each increase in temperature of 33 F.

The solution to the above formula may be as exacting as desired depending upon the users mathematical talents and available mathematical equipment as well as the characteristics of the users heating equipment and control for the heating equipment. It is pointed out, however, that the objective is to diffuse only a portion, such as 25% to 50%, of the available delta phase constituent which may well vary from, for instance, 2% to 7% from bronze specimen to bronze specimen. It has been found that the percentage of delta phase constituent in S.A.E. 640 continuous cast bronze will vary from supplier to supplier and even from piece to piece within a batch supplied by a particular supplier. Since this condition exists, it may not be worth while very precisely to solve the above integration formula.

From a practicability standpoint, therefore, it is advisable to determine the time-temperature profile for given specimens in given heating devices to achieve the desired reduction in delta phase. By testing the specimens, it can be determined whether a time-temperature profile is suitable for purposes of diffusing the proper amount of delta phase. For instance, speciments can be roll tested to determine whether the surface fatigue properties are increased by the time-temperature profile. Of course, metallographic studies to determine the delta phase content may be used to supplement the roll testing. DEP calculations, or total diffusion calculations, made for specimens which are significantly improved will establish a basis for determining the time-temperature profile for other specimens.

The durability or wearability of a transmission or bearing element is an extremely important and often critical factor. Elements formed from material which is more durable can, obviously, carry heavier loads for greater lengths of time than elements which are fabricated from less durable material. A material which is more durable will have a higher load-stress K factor than a material which is less durable. For a discussion of such factors, we refer to an article titled Wear Life Of Rolling Surfaces starting at page 44 in the May 9, 1960 issue of Product Engineering and to Buckingham, Analytical Mechanics of Gears, 1963 Dover Edition, Chapter 23.

We have constructed in accordance with the present invention and tested steel-bronze composite rollers, i.e., a

bronze ring soldered to a steel hub. In addition, as a com-.

parison, we tested solid bronze rollers which had not been subjected to the heating and cooling involved in soldering the bronze ring to the steel hub. In these tests, we found that the steel-bronze composite rollers had experimental load-stress factors, as discussed in the above-referred article and book, more than twice that of the solid bronze, non-heat-treated rollers. We noted that the heating and cooling steps did not, significantly, change the hardness of the bronze. The following is a breakdown of the samples which were fabricated and tested in this effort:

11 1 SOLID BRONZE ROLLERS Material: S.A.E. 640 continuously cast bronze formed to provide a solid bronze roller, but not heat treated.

Hardness: 109-1 14 BHN.

Load factor: 607.

FIRST STEEL-BRONZE COMPOSITE ROLLERS Material: Steel hub with an S.A.E. 640 bronze ring 1 soldered thereto, the ring being sliced from a continuously cast bronze sleeve.

Process of fabrication: Each bronze ring was soldered to its steel hub by the induction heating process, supra, air cooling the ring and hub to 800 F. and then water quenching.

Hardness: 102-107 BHN (after such heating and cooling).

Load factor: 1 363.

SECOND STEEL-BRONZE COMPOSITE ROLLERS Material: Steel hub with an S.A.E. 640 bronze ring soldered thereto, the ring being sliced from a continuously cast bronze sleeve.

Process of fabrication: Each bronze ring was soldered to its steel hub by the muflle furnace process, supra, air cooling the ring and hub to 800 F. and then Water quenching.

Hardness: 99-102 BHN (after such heating and cooling).

Load factor: 1343.

With the above-established results in mind, we then constructed (in accordance with the method of the present invention) and tested steel-bronze composite rollers and, for comparison purposes again, we fabricated and tested solid bronze rollers which were subjected to the heating and cooling steps involved in our method for joining bronze rings to steel hubs. The results'of these tests indicated that the heating and cooling steps, such as those involved in our above-described soldering process, significantly improve the wearability of the bronze. Without undue elaboration, we can say that these tests have proved that, when S.A.E. 640 bronze is heated and then subsequently cooled as discussed in conjunction with the description of our soldering processes, the load factor K is improved to the point where it is 2.77 times greater than the load factor K for conventional chill cast bronze which is not heat treated.

Further, we have tested steel-composite gears fabricated using our soldering methods. That is, we have life tested actual gear boxes comprising composite gears fabricated by our methods. These life tests of gear boxes have led us to believe that the durability or wearability tests made with the rollers and described previously are applicable directly to gear box testing.

The bronze ring had an CD. of 2.93 inches, an ID. of 1.7 Inches and a thickness of .75 inch.

What is claimed is:

1. The method of improving the wear characteristics of a copper-tin bronze cast to include a substantial delta phase constituent in an alpha phase matrix including the steps of heating the bronze to a selected elevated temperature range within the diffusion range of 968 F. to 1470 F., holding the bronze within said elevated temperature range for a predetermined time, and then cooling the bronze to diffuse from approximately to 50% of the delta phase constituent into the alpha phase matrix, said heating, holding and cooling times conforming to the following formula:

elevated temperature range Total diffusion: RDF T wherein RDF is the relative diffusion factor based upon the temperature-dependent diffusion rate of delta phase constituent into the alpha phase matrix, and T T and T are, respectively, the heating, holding and cooling times.

RDF T levated temperature range 2. The method of claim 1 in which said selected elevated temperature range is within the range of approximately 1000 F. to 1260 F.

3. The method of claim 2 in which said selected elevated temperature range is within the range of approximately 1100 F. to 1200 F.

4. The method of claim 2 in which said heating step includes placing the bronze in a furnace held at approximately 1400 F. and said holding time T is approximately 15 seconds. 9

5. The method of claim 4 in which said selected elevated temperature range is within the range of 1100 F. to 1200" F.

6. The method of improving the wear characteristics of a copper-tin bronze cast to include a substantial delta phase constituent of crystals having sharp corners and sharp edges in an alpha phase matrix including the steps of heating the bronze to a selected elevated temperature range within the diffusion range of 968 F. to 1470 F., holding the bronze within said elevated temperature range for a predetermined time, and then cooling the bronze to diffuse said sharp corners and sharp edges into the alpha phase matrix while leaving at least approximately 50% of the total delta phase constituent present in the bronze as cast, said heating, holding and cooling times conforming to the following formula:

elevated temperature ran e Total diffusion: 968 F D RDFX T;

levated temperature range wherein RDF is the relative diffusion factor based upon the temperature-dependent diffusion rate of delta phase constituent into the alpha phase matrix, and T T and T are, respectively, the heating, holding and cooling times.

7. The method of claim 6 in which said selected elevated temperature range is within the range of approximately 1000 F. to 1260 F.

8. The method of claim 7 in which said selected ele vated temperature range is within the range of approximately 1100 F. to 1200 F.

9. The method of improving the pitting wear resistan-ce of a copper-tin bronze cast to include at least 2% delta phase constituent in an alpha phase matrix without significantly deteriorating the abrasive wear resistance of such bronze which method comprises the steps of heating the bronze to a temperature between 968 F. and 1470 F. and subsequently cooling the bronze, by exposing the same to atmospheric conditions, to a temperature not exceedng 968 F., thereby smoothing the outer surfaces of the particles of delta phase constituent present by diffusion into the matrix, and so controlling the total time during which the temperature of the bronze remains within the claimed range such that at least 25% but not more than 50% of the delta phase constituent is so diffused.

10. The method of improving the pitting wear resistance of a copper-tin bronze cast to include at least 2 delta phase constituent of crystals having sharp corners and sharp edges in an alpha phase matrix without significantly deteriorating the abrasive wear resistance of such bronze which method comprises the steps of heating the bronze to a temperature between 968 F. and 1470 F. and subsequently cooling the bronze to a temperature below 968 F., holding the temperature of the bronze within the claimed range for a total period sufficient to diffuse off said sharp corners and sharp edges while leaving at least 50% of the total delta phase constituent present in the bronze as cast.

11. The method of improving the wear characteristics of copper-tin bronze with sufficient tin to form a substantial delta phase constituent as cast which comprises the steps of heating the bronze to a selected elevated temperated range within the diffusion range of 968 F. to 1470 F., holding the bronze within said elevated tem- 13 perature range for a predetermined time, and then cooling the bronze to diffuse from approximately 25% to approximately 50% of the delta phase constituent into the alpha phase matrix, the holding time conforming to the following formula:

wherein T is the holding time, RDF is the relative diffusion factor Within said elevated range and D is the desired diffusion occurring during the holding time.

12. The method of claim 11 in which said selected elevated temperature range is within the range of 1000 F. to 1260 F.

13. The method of claim 12 in which said selected elevated temperature range is within the range of 1100 F. to 1200" F.

References Cited UNITED STATES PATENTS 1,338,672 5/ 1920 Calkins 74-446 1,766,865 6/1930 Williams et al.

2,157,918 5/1939 Rankin 29-504 X 2,190,267 2/ 1940 Light 75-154 2,231,014 2/ 1941 Lytle et al. 29498 X 2,231,427 2/1941 Larsh et al.

14 2,645,006 7/1953 Hadley 29504 X 2,709,375 5/1955 Sandberg 291592 X 2,756,607 7/ 1956 Mochel.

3,290,182 12/1966 Eichelman 148-127 X 3,461,738 8/1969 'Pandjiris et al. 29159.2 X

FOREIGN PATENTS 1,085,988 10/1967 Great Britain. 869,033 5/1961 Great Britain. 757,936 9/1956 Great Britain. 512,142 8/ 1939 Great Britain. 486,600 6/1938 Great Britain. 413,333 7/1934 Great Britain.

OTHER REFERENCES Chill-Cast Tin Bronzes, Arnold & Co., London, 1951, Heat Treatment of Cast Bronzes, pp. 295-298, 300, 302-305 and 308415.

CHARLES N. LOVELL, Primary Examiner US. Cl. X.R. 148-160 Pww UNITEDSTATES PA'IENE dFFIcE (TIERTIF [GATE 0 F (.4 R HEW! l ON Patent No. 3,824,132 Dated July 16 19-74 lnventor(s) Robert L. Wolfe and Augustine Trebnik It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

" Column 1, line 57, "or" should be our Column 3, line 9, constituent is misspelled. Column 6 line 7, "joints" should be joins cancel line 34; last line, thereafter is misspelled.

Column 7, cancel line 9 and substitute melt the solder and to cause the molten solder to flow Column 8, line 8, "986 F." should be 968 F.

Column 9, line 28, "986 F." should be 968 F. Column 10, line 42, specimens is misspelled.

Signed and sealed this 29th day of October 1974.

(SEAL) Attest:

McCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents 

